High-temperature ovens, or reactors, are used to process semiconductor wafers from which integrated circuits are made for the electronics industry. A substrate, typically a circular silicon wafer, is placed on a wafer holder. If the wafer holder helps to attract heat, it is called a susceptor. The wafer and wafer holder are enclosed in a quartz chamber and heated to high temperatures, such as 600-1200° C. or even higher, by a plurality of radiant lamps placed around the quartz chamber. A reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. Through subsequent processes in other equipment, these layers are made into integrated circuits, with a single layer producing from tens to thousands of integrated circuits, depending on the size of the wafer and the complexity of the circuits. In recent years, single-wafer processing has grown for a variety of reasons, including its greater precision as opposed to processing batches of wafers at the same time, while larger diameter wafers are employed to compensate for reduced throughput as compared to batch processing.
Typically, a wafer is inserted into a reaction chamber through the use of a robotic end effector. In one arrangement, the end effector comprises a Bernoulli wand, which operates on the Bernoulli principle; it provides a plurality of relatively high velocity gas flow streams above the wafer, creating a pressure drop across the wafer to draw it upward toward the wand. The jet streams are normally angled slightly downward to prevent direct contact between the wafer and the wand. A typical gas for use within a Bernoulli wand is N2. The Bernoulli wand includes a lower wand foot that prevents the wafer from flowing laterally away from the wand, the wand foot normally extending downwardly below the lower surface of the wafer. The Bernoulli wand typically centers the wafer above the wafer holder and then either lowers the wafer onto the wafer holder or permits the wafer to drop thereon. The wafer holder may include a pocket or recess designed to receive the wafer. The top surface of the pocket of the wafer holder may include grooves to permit gas between the wafer and the wafer holder to escape around the edges of the wafer. Such grooves help to prevent the wafer from sliding horizontally with respect to the wafer holder.
In another arrangement, the robotic end effector comprises a forked member that supports the lower surface of the wafer only at the wafer's outer edges. Such an end effector is shown, for example, in U.S. Pat. No. 6,293,749 to Raaijmakers et al. In that invention, the end effector includes two arms that define an inner clearance wide enough to accommodate the vertical movement of an inner wafer holder section. Wafer loading is accomplished by supporting the wafer on the end effector, moving the inner wafer holder section vertically through the clearance defined by the two arms of the end effector to lift the wafer thereabove, withdrawing the end effector from the reaction chamber, and then lowering the inner wafer holder section. Alternatively, lift pins may be vertically raised to receive the wafer from the fork-type end effector.
Various CVD process parameters must be carefully controlled to ensure the high quality of the resulting semiconductor. One such critical parameter is the temperature difference between the wafer and the wafer holder as the former is loaded onto the latter. As explained above, CVD processing often occurs at temperatures of 600-1200° C. or even higher. A common problem associated with CVD processing is “thermal shock,” which can result in wafer “curl” or “pop.” When a relatively cold (e.g., room temperature) wafer is loaded onto the top surface of a relatively hot (e.g., 600° C. or higher) wafer holder, the wafer can experience thermal shock due to thermal gradients within the wafer, caused by sudden conductive heat transfer from the wafer holder to the wafer. These thermal stresses can cause the wafer to curl, i.e., to deform by bending, often decentering the wafer relative to the wafer holder. In extreme cases, the wafer may pop, which can cause the wafer to jump out of the pocket of the wafer holder and possibly damage the end effector. Contact between the wafer and the end effector can also damage the wafer topside and/or cause particles to flake off of the wafer and contaminate the chamber. Thermal shock can also result in crystallographic slip. Slip is a defect in the crystalline structure of the wafer, which destroys any devices through which it may pass. Curl, pop, and slip typically degrade the performance of the wafer (e.g., increased device and current leakage) and can even render the wafer unusable.
Several methods can be employed to reduce thermal shock. One solution is to reduce the temperature of the reaction chamber and wafer holder prior to the insertion of each new wafer. In this method, the temperature of the wafer holder is reduced to a level such that the wafer is unlikely to curl when it is brought into contact with the wafer holder. Once the wafer is loaded, the temperature of the reaction chamber, including the wafer holder and wafer, is steadily increased to a desired processing temperature. While this method of controlling the temperature of the reaction chamber can prevent thermal shock, it significantly reduces throughput, as it is very time consuming to continually vary the temperature of the reaction chamber between the higher processing temperature and the lower temperature at which thermal shock is substantially prevented. Since reduction in throughput results in lower production and higher production costs, this method of reducing thermal shock is rarely used in practice.
Another solution for preventing thermal shock is to gradually preheat the wafer before it is brought into contact with the wafer holder. Typically, the wafer is inserted into a preheated reaction chamber and, for some time, held above the wafer holder without any contact therebetween. While positioned above the wafer holder, the wafer receives heat in the form of radiation from the lamps surrounding the reaction chamber and convection from warm gas within the chamber. The wafer is maintained in such position until the wafer temperature rises to a level at which thermal shock is unlikely to occur should the wafer be lowered onto the wafer holder. Once the wafer reaches that temperature, the wafer is lowered onto the wafer holder. This method is disclosed in co-pending U.S. patent application Ser. No. 09/840,532, entitled “High-Temperature Drop-off of a Substrate,” filed Apr. 23, 2001.
In one method of preheating a wafer, the wafer is held above the wafer holder by a robotic end effector for a few seconds before being dropped onto the wafer holder. In another method, the wafer is held above the wafer holder by a plurality of vertically movable lift pins arranged about the circumference of the wafer. The lift pins typically provide support to the wafer at three or more positions near the wafer's outer radial periphery. The lift pins have a raised position in which the wafer does not contact the wafer holder and a lowered position in which the wafer rests upon the wafer holder. When the wafer is inserted into the reaction chamber, the lift pins are raised so that the wafer does not contact the wafer holder. Once the wafer temperature increases sufficiently, the lift pins are lowered so that the wafer is brought into contact with the wafer holder.
After a wafer is processed and removed from a reaction chamber and before a new wafer is inserted therein for processing, a purge gas such as N2, H2, or noble gas is typically directed through the chamber. The purge gas helps to prevent particles that may be present in the wafer handling chamber and other areas from entering and possibly contaminating the reaction chamber. The purge gas also prevents undesired oxidation on the surface of a wafer as it is inserted into the reaction chamber.